1. Field of the Invention
The present invention relates to a friction stir welding method according to which the workpieces to be welded are positioned on a work-table and by means of clamping means clamped to one another and/or to the worktable and according to which a rotating welding means is arranged to move along a joint between the workpieces while being pressed against said workpieces during the welding.
The present invention likewise relates to an apparatus for friction stir welding, comprising a worktable supporting the workpieces to be welded, at least one clamping means for clamping the workpieces to one another and/or to the work-table, and a welding means adapted to be advanced along a joint between the workpieces while being pressed against said workpieces during the welding.
2. Description of Related Art
The definition friction stir welding designates a welding method according to which the workpieces to be joined together are plasticized along their joint line by supply of frictional heat from a rotating welding probe, said probe being moved along the joint between the workpieces while at the same time being pressed against the work-pieces. As described in WO93/10935 and WO95/26254 the welding probe should be manufactured from a material that is harder than that of the workpieces, and during the welding operation the workpieces should be securely fixed relative to one another and to the worktable. In this respect this technique differs from that of conventional friction welding according to which the frictional heat is generated by the relative motion of the workpieces as they are being pressed together, i.e. the frictional heat is generated only by the two components to be joined together. This friction stir welding technique according to the above-mentioned publications is used for the welding of different materials, such as plastics, metals, etcetera in various fields of application, for example to join workpieces together, to repair cracks in a workpiece, and so on. The design of the welding probe is conditioned e.g. by the material to be welded and by the intended application.
In gas metal arc welding and particularly in submerged arc welding when the molten pools generally are of considerable dimensions, a backing means is often used, i.e. a backing bar which is pressed against the back of the joint during the welding operation in order to support the edges of the joint and to prevent the molten pool from escaping as the complete weld penetration stage is reached. In constructing the backing means thermal considerations-are to a large extent decisive in the choice of the backing means material, particularly in the case of welding of aluminum. The cooling effect on the joint from the backing means must not be too powerful.
Similarly, backing means are likewise used in complete penetrating welding in connection with friction stir welding. In this case, the backing means serves as a mechanical support and shapes the lower face of the joint. Also in this case the backing means may be formed with a thermal barrier against a subjacent backing bar in order better to take advantage of the friction heat generated in the welding process.
One of the problems encountered in friction stir welding of hard-to-weld materials, such as aluminum alloyed with magnesium, with lithium or with copper, is that the forces deployed during the welding operation are of such a magnitude as to cause the welding probe to break after a comparatively short period of welding, as a result of fatigue. This is true particularly of workpieces that have a thickness exceeding 5 mm.
Another problem is the generation of short but deep cracks in the surface of the backing means, particularly in the transverse direction of the latter, due to thermal fatigue. Because the joint, owing to the plasticisation thereof during the welding operation, will adopt the appearance of the backing means surface, these cracks will manifest themselves as protrusions on the lower face of the joint, and as a result the joint will be unacceptable.
A further problem is that the frictional heat generated in hard-to-weld materials may be insufficient, and that consequently complete weld penetration is not achieved, or that lack of fusion will occur.